The present invention relates generally to a light source provided with a semiconductor light emitting device. The light source is widely used for various kinds of electronic apparatuses such as an optical disk apparatus, a copy machine, a printer, a lighting apparatus, optical communication application, and a laser display.
A semiconductor laser formed from a III-V group nitrogen semiconductor material (AlxGayIn1-x-yN (where 0≦x≦1, and 0≦y≦1)) is a key device for realizing ultra high density recording in an optical disk apparatus. Presently, as a light source required for data recording with higher density than DVD, a GaN blue-violet semiconductor laser is the closest one to the practical level. Increase in power of the blue-violet semiconductor laser enables high-speed writing to an optical disk to realize, and additionally, the increase in power of the blue-violet semiconductor laser is an essential technique for pioneering new technical fields such as an application to a laser display.
Hereinafter, with reference to FIGS. 7A and 7B, a prior-art blue-violet semi-conductor laser will be described. The semiconductor laser 801 shown in the figures includes a substrate 701 and a multilayer structure formed on the substrate 701. The multilayer structure includes, from the side of the substrate 701, an n-AlGaN cladding layer 702, a quantum well active layer 703, a p-AlGaN cladding layer 704, and a p-GaN contact layer 705. In an upper portion of the semiconductor multilayer structure, the p-AlGaN cladding layer 704 and part of the p-GaN contact layer 705 are processed so as to have a stripe shape, so as to form a ridge stripe 706 for current confinement. Both sides of the ridge stripe 706 are covered with an insulating layer 707. A p-electrode 708 is formed on a top face of the ridge stripe 706, and an n-electrode 709 is formed on a back face of the substrate 701.
In the operation, according to an increase in current injected from the p-electrode 708 and the n-electrode 709, a carrier density in the quantum well active layer 703 is increased. When the value reaches a predetermined threshold carrier density, laser oscillation is obtained.
In a rewritable optical disk apparatus, a high-power semiconductor laser is desired. A conventional technique is used in which reflectivities of two end faces constituting a cavity (resonator) of a semiconductor laser are asymmetric for the purpose of realizing higher power.
In a semiconductor laser used for writing to an optical disk, cavity end faces are coated with dielectric multilayer films, so that reflectivities of the end faces are made to be asymmetric. One of the cavity end faces on the side from which laser light is emitted (a light emitting end face) is made to have a lower reflectivity, and the end face on the other side (a back end face) is made to have a higher reflectivity. For example, the reflectivity of the light emitting end face is set to be 10%, and the reflectivity of the back end face is set to be 90%. The reflectivity of the dielectric multilayer film can be controlled by a refractive index and a thickness of a dielectric layer to be deposited, and the number of layers to be stacked.
The semiconductor laser 801 shown in FIG. 7A is packaged in a can package (container) shown in FIG. 8A, and used as a light radiating element of short-wavelength light. The package (a short-wavelength light source) includes a base 803 and a cap 804. The semiconductor laser 801 and a sub-mount 802 functioning as a radiator are mounted on the base 803. The cap 804 includes a glass plate 806 functioning as a light transmitting window for taking out the light, and a metal foundation (can) 805. The semiconductor laser 801 is mounted on the base 803 via the sub-mount 802. In the base 803, openings for terminals are disposed, and the terminals are fixedly attached by a low-melting glass 807.
In order to maintain the air-tightness in the package, a gap between the glass plate 806 and the can 805 is closed by a low-melting glass 808 (fixed at several hundreds of degrees), as shown in FIG. 8B. An internal space enclosed by the base 803 and the cap 804 is filled with a nitrogen (N2) gas or the like.
However, the short-wavelength light source shown in FIG. 8A causes a problem that when the semiconductor laser 801 operates with high optical output power of about 30 mW for a long period of time, a foreign material is elliptically deposited on the light emitting end face of the semiconductor laser 801.
It is found by elemental analysis (mass spectrometry such as EDX) that the foreign material is a material mainly including carbon (C) or silicon (Si). It is also found that the deposition of the foreign material is increased in accordance with the increase in optical power of the semiconductor laser 801. Therefore, the phenomenon of the deposition of the foreign material is a serious problem for increasing the power of the light source and for realizing high-speed recording to a rewritable optical disk apparatus.
According to experiments by the inventors of the present invention, it is also found that the deposition of foreign material is not only caused in the inside of the package shown in FIG. 8A. Specifically, in various electronic apparatuses (an optical pickup apparatus, for example) provided with a short-wavelength semiconductor laser with an oscillation wavelength of 450 nm or less, it is observed that the foreign material is deposited on a portion irradiated with laser light (especially in a portion with higher optical density). On the contrary, since the phenomenon of the deposition of the foreign material is not observed in other semiconductor lasers (a red laser, or an infrared laser), it is considered that the phenomenon remarkably occurs in short-wavelength semiconductor lasers with oscillation wavelength of 450 nm or less. In addition, such a phenomenon may cause even in a visible range of wavelengths when the optical power is increased.
The C and Si deposited in the package shown in FIG. 8A can be derived from various materials of slight amounts existing in the air (hydrocarbon or siloxane). For this reason, it is extremely difficult to perform an assembly process in a condition where deposition causative materials such as C or Si are not mixed in the package. In addition, it is impossible in reality to prevent the causative materials existing in the air from entering the optical disk apparatus. Even if the inside of the package can be held in a condition of C-free or Si-free, it is impossible to prevent the materials including C or Si from depositing on optical components such as a lens which is irradiated with short-wavelength laser light emitted from the light source.